1. Field of the Invention
The present invention relates to an optically pumped solid-state laser oscillator, and more particularly to a diode-pumped solid-state laser oscillator in which the output-power dependency on the temperature of the pumping laser diodes and the laser medium can be decreased.
2. Description of the Prior Art
In space satellites, aircraft, road vehicles and other such means of transportation, there are often limits on the amount of electrical power that can be used. Due to this limitation, laser oscillators required for stable output power for such mobile applications are usually excited by laser diodes (LDs) that have high efficiency.
However, as described in further detail below, the output power of such diode-pumped solid-state lasers varies due to the temperature dependency of the emission wavelength of the laser diodes. Changes in the temperature can change the pump-light absorption index of the laser rod used, altering the laser output power.
The normal temperature dependence of the wavelength of a pumping LD is 0.27 nm per degree centigrade. This means that in the case of an LD that has an operating temperature range of 50° C., the emission wavelength can change by 14 nm. The absorption spectrum of a laser rod used in a diode-pumped solid-state laser consists of narrow spectral lines corresponding to the distribution of the excited energy levels of the Nd ion or other such laser active element used. FIG. 5 shows the absorption spectrum of a Nd:YAG laser crystal plotted with the resolution of 4 nm which is the same as the usual wavelength width of LD arrays. In the case of wavelengths of 802 to 817 nm that include an absorption peak at 808 nm, as in FIG. 5, the absorption coefficient changes from 3.8 cm−1 to 1.2 cm−1. If the propagation length of the pump light in the Nd:YAG crystal is 8 mm, the absorption index will vary from 95% to 62% within these wavelengths, as shown in FIG. 6.
In the prior art, lasers are designed to obtain optimal laser oscillation, for example, taking the pumping center wavelength to 808 nm at the absorption peak of the laser crystal. Therefore, wavelength deviation of LDs arising from the changes in temperature results in a decrease in pump-light absorption, reducing the output power. In order to prevent such reduction, it has been necessary to maintain the LDs at a constant temperature, for which controlling mechanism needs to be installed. However, it consumes a relatively large amount of electrical power compared to the pumping power in controlling the temperature of a pump-light source or laser medium, so when electrical power is limited, it is difficult to accomplish such a control.
In the case of the air-cooled diode-pumped laser shown in FIG. 9, for example, the cooling airflow at the radiation fins is controlled to stabilize the temperature of the LDs. This means that electrical power is necessary to cool and to stabilize the temperature of the cooling fins. Moreover, vibrations and noises generated by a cooling fan perturb the laser resonator, reducing the stability of the output power. In addition, the temperature control has a high time constant that makes it difficult for the system to respond quickly to the temperature changes. 
In the case of a water-cooled diode-pumped laser known to the inventor, cooling water controlled to a designed temperature is circulated to maintain the temperature of the LDs. This cooling apparatus is bulky and consumes a lot of electrical power. Here too, vibrations from the pumps used in the cooler and circulator are large and result in a loss of output power stability.
In the an additional known small power diode-pumped laser, the LDs are cooled using a Peltier cooling element. Since the cooling efficiency of the Peltier element is not high, in an environment where there is limited electrical power, such an element can be used only in small power lasers.
The diode-pumped Nd:YAG laser oscillator for use in the laser altimeter installed in NASA's Mars Observer spacecraft has a partial resemblance to the present invention. This laser oscillator is described in a paper (Robert Afzal, “Mars Observer Laser Altimeter,” Applied Optics vol. 33, No. 15, pp 3184–3188 (1994)) that also includes a graph showing the relationship between the laser oscillator output power and the operating temperature. In the laser oscillator, the pumping source is four diode-laser stacks of 11 diode bars having a narrow wavelength width of 6 nm. The result is a range of operating temperatures that is wider than that obtained using pumping at just one wavelength. However, this laser oscillator differs from that of the present invention in that it does not use the optical absorption characteristics of the laser medium and the temperature characteristics of the pumping source.
As described in the foregoing, although in the prior art diode-pumped solid-state laser oscillators are used where there is limited electrical power, they have a number of problems, such as that electrical power is required for cooling, that air cooling has a high time constant and vibration produced by the cooling system also makes it difficult to maintain a stable output, that water cooling requires a large apparatus and the vibration has an adverse effect on output stability, and that cooling systems that use a Peltier element do not generate vibration but do require a lot of electrical power.
In view of the above, an object of the present invention is to provide a diode-pumped solid-state laser oscillator that is able to provide a stable laser output power using less electrical power for temperature control.